Wray Herbert notes the fallacy of interpreting fMRI and other brain imagery as especially meaningful: "The Brain Is Not an Explanation". I'm pointing to this because of the similarities between brain scanning in neuroscience and genotyping in genetics. The result certainly looks objective, but what is actually there?
The problem is that the final product—the brain image—looks like a photograph, and that’s how most readers take it, as a simple snapshot of the brain in action. That’s in part because the simplicity of the message is appealing: Complicated behavior X lights up brain area Y. But such reductionism, Beck argues, lacks any explanatory power. Consider the chocoholic example again: Leaving aside the fact that chocoholic is not a recognized diagnosis, what does this study actually show? It shows that people who define themselves as chocolate cravers have more activity, relative to people who do not define themselves as chocolate cravers, is certain pleasures centers of the brain. That is, the sight and taste of chocolate activated the brain’s reward system in cravers, documenting . . . what? Well, documenting that some people find chocolate more rewarding than others. As Beck notes, we probably don’t need a brain scan to corroborate what most people probably already believe anyway.
A study would be powerful if it gave a way of predicting phenotypes or behaviors from the scans (or the genotypes). But finding a correlate of a behavior doesn't make it more real than it already is. Remember that we can already predict many phenotypes from family history much more accurately than we can from genotypes. As Galton discovered, the additive genetic component of variance exists even if we know nothing about the mechanisms of transmission.
But then, if the modality didn't matter, we wouldn't need to bother with chocolate at all. We could just stimulate the pleasure center directly.